The stencil printing system has heretofore been broadly practiced as a simple printing system. According to this stencil printing system, a laminate having a thermoplastic film layer laminated on an appropriate support (base) surface such as paper, etc. is used as the heat-sensitive stencil sheet. By performing printing by a heating printing means such as thermal heads on the paper, the thermoplastic film layer is melted by heating to form a perforated pattern corresponding to the printed information for printing. In stencil printing, the film layer side of the paper having printing thus effected thereon, is superposed on a material to be printed such as paper, etc. and a printing ink is fed in from the base side.
The heat-sensitive stencil sheet to be used in the heat-sensitive stencil printing system of the prior art as described above is generally formed by laminating a thin thermoplastic film layer of about several .mu.m on the surface of a porous base such as paper, etc. with an adhesive, etc. but the following problems are involved.
(1) Since the adhesive layer is also required to be thermoplastic for enabling perforation with heat, no strong adhesive such as a thermosetting adhesive can be used, and therefore no sufficient adhesive force can be obtained. Also, for the same reason, during printing, the adhesive force will be lowered due to the solvent components, etc. in the printing ink, whereby there is the problem that poor print quality may occur if the thermoplastic film peels off from the layer due to the thermal head, etc. Thus, the heat-sensitive stencil sheet of the prior art has the problem that it is not satisfactory with respect to printing resistance.
(2) Since an organic solvent solution or emulsion of a thermoplastic resin is generally used as the adhesive, the required drying step and aging step make the process cumbersome, and also increase the cost. Also, the heat required in the drying and aging steps causes thermoplastic film to be deformed to make a stable paper product difficult to achieve Further, a solvent system, emulsion system adhesive, through impregnation into the porous support will clog the voids in the porous base, thereby having the drawback of being inferior in image quality or image density.
(3) As the result of use of a porous material such as paper, etc. as the base, it is necessary to enhance the adhesive force to some extent in order to adhere both sufficiently to each other. However, when the adhesive force between the support and the film layer becomes higher, the surface unevenness shape on the porous support will be transferred onto the surface of the thin thermoplastic film layer, whereby there is the problem of nonuniformity occurring on the printed image as caused by the uneven shape during printing.
An object of the present invention is to solve the drawbacks as described above, and provide an excellent heat-sensitive stencil sheet economically.